B737 Tailwind Operation
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B737 Tailwind Operation
Hello friends
First of all I would like to request you to read that few lines below in order to help you understand my question.
3.5.2 Tower Wind Reporting
First of all I would like to request you to read that few lines below in order to help you understand my question.
3.5.2 Tower Wind Reporting
On short final and just before takeoff the pilot may obtain a wind report from the tower. This
wind report is also based on the same wind sensors as for the ATIS wind. However the tower
wind report is more accurate than the ATIS report since it is based on a two-minute period
preceding the pilot's contact with the control tower. As is the case for ATIS information, tower
wind reports need to be decomposed into cross- and tailwind components by the flight crew.
wind report is also based on the same wind sensors as for the ATIS wind. However the tower
wind report is more accurate than the ATIS report since it is based on a two-minute period
preceding the pilot's contact with the control tower. As is the case for ATIS information, tower
wind reports need to be decomposed into cross- and tailwind components by the flight crew.
3.5.3 FMS Wind
The FMS wind is computed as a vectorial difference between the airspeed aligned to the aircraft
heading and the ground speed aligned to the ground track. The FMS-calculated wind vector is
normally displayed on the Navigation Display (ND) or on the Control Display Unit (CDU).
Some FMS installations provide a decomposition of the wind vector in cross- and tailwind
components. The FMS calculates the wind for the altitude the aircraft is actually flying. Note
that tailwind limits and the tailwind used for field performance refer to the wind measured at a
10-meter height. The FMS wind is therefore of little value to the pilot when he makes his
decision to land, i.e. at top of descent, during descent and upon initiation of the final approach.
Nevertheless many pilots tend to monitor the FMS for exceedance of the maximum tailwind.
Uncertainties exist in the determination of derived inertial quantities (like ground speed and
ground track) that will influence the accurate determination of the FMS wind vector. Especially
the calculation of the drift angle should be treated with suspect in a dynamic environment like
an approach. Secondly, the airspeed is assumed to be aligned with the heading, sideslip is not
measured nor incorporated in the calculation of the FMS wind, yielding questionable results
once the aircraft has commenced decrabbing in crosswind conditions that might be present as
well. Finally, FMS computations are filtered, resulting in a typical time delay of 3-5 seconds. A
second relevant effect of this filtering process is that gust values will not be displayed to the
flight crew. For these reasons the use of FMS wind is normally accurate only in the cruise phase
of the flight. However, it can be shown that, although the crosswind component determined by
the FMS can be highly inaccurate in the final phase of the flight, the tailwind component is
relatively insensitive to FMS errors in the determination of the drift angle. This is a direct result
of the geometry of the speed vectors involved.heading and the ground speed aligned to the ground track. The FMS-calculated wind vector is
normally displayed on the Navigation Display (ND) or on the Control Display Unit (CDU).
Some FMS installations provide a decomposition of the wind vector in cross- and tailwind
components. The FMS calculates the wind for the altitude the aircraft is actually flying. Note
that tailwind limits and the tailwind used for field performance refer to the wind measured at a
10-meter height. The FMS wind is therefore of little value to the pilot when he makes his
decision to land, i.e. at top of descent, during descent and upon initiation of the final approach.
Nevertheless many pilots tend to monitor the FMS for exceedance of the maximum tailwind.
Uncertainties exist in the determination of derived inertial quantities (like ground speed and
ground track) that will influence the accurate determination of the FMS wind vector. Especially
the calculation of the drift angle should be treated with suspect in a dynamic environment like
an approach. Secondly, the airspeed is assumed to be aligned with the heading, sideslip is not
measured nor incorporated in the calculation of the FMS wind, yielding questionable results
once the aircraft has commenced decrabbing in crosswind conditions that might be present as
well. Finally, FMS computations are filtered, resulting in a typical time delay of 3-5 seconds. A
second relevant effect of this filtering process is that gust values will not be displayed to the
flight crew. For these reasons the use of FMS wind is normally accurate only in the cruise phase
of the flight. However, it can be shown that, although the crosswind component determined by
the FMS can be highly inaccurate in the final phase of the flight, the tailwind component is
relatively insensitive to FMS errors in the determination of the drift angle. This is a direct result
Thank you very much for your patience and the question is...
Is there any company that uses the FMS Wind information during the final approach (short final) in order to make a decision of landing or not, disreguarding the Wind reported by the Tower ???
Thanks and looking forward any answer.
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Our SOP's say always use the tower wind for decision regarding landing winds and associated calculations. Were not allowed use the FMC wind on final approach for anything other than information.
This topic comes up regularly and always gets people going. In my company (737NG) we encourage pilots to have progress page 2 open, with the tailwind component visible. I believe this is good airmanship.
As has been pointed out in previous threads there appears occasionally to be some dubious reporting of winds at certain southern european airfields. Then again there is no doubt that the tower reported wind takes priority for legal purposes. As with everything it is a question of common sense.
If the tower is reporting a wind within limits and you are landing on a dry 3000 metre runway, then it shouldn't be necessary to go around if the FMS is showing 11 knots tailwind.
On the other hand if you are landing in a heavy rain shower at a small airport in central France it may make sense to err on the side of caution, as recent history has shown.
The FMS wind is but part of the whole picture. Its limitations need to be understood. But if the runway is short and wet and the FMS is telling you something different from the tower then you should be on your guard. Better one go-around too many than one too few!
As has been pointed out in previous threads there appears occasionally to be some dubious reporting of winds at certain southern european airfields. Then again there is no doubt that the tower reported wind takes priority for legal purposes. As with everything it is a question of common sense.
If the tower is reporting a wind within limits and you are landing on a dry 3000 metre runway, then it shouldn't be necessary to go around if the FMS is showing 11 knots tailwind.
On the other hand if you are landing in a heavy rain shower at a small airport in central France it may make sense to err on the side of caution, as recent history has shown.
The FMS wind is but part of the whole picture. Its limitations need to be understood. But if the runway is short and wet and the FMS is telling you something different from the tower then you should be on your guard. Better one go-around too many than one too few!
The FMS is of little value in assessing wind for landing (background or trend info maybe); see page 15 of the report - “Safety aspects of tailwind operations”.
Also, note that although the reported tower wind is more accurate than the FMS it is subject to some significant error, which could place the aircraft outside of the tail or crosswind operating limit. See the report “Safety aspects of aircraft operations in crosswind”.
“… that a reasonable probability does exist, that while wind reports to the pilot do indicate that crosswind is not exceeding 15 Kt, in reality the actual encountered crosswind during the landing phase can deviate 10 Kt or even more from the reported wind.
For even higher reported crosswinds, deviations may increase accordingly.”
Tailwind reports could have similar errors.
Also, note that although the reported tower wind is more accurate than the FMS it is subject to some significant error, which could place the aircraft outside of the tail or crosswind operating limit. See the report “Safety aspects of aircraft operations in crosswind”.
“… that a reasonable probability does exist, that while wind reports to the pilot do indicate that crosswind is not exceeding 15 Kt, in reality the actual encountered crosswind during the landing phase can deviate 10 Kt or even more from the reported wind.
For even higher reported crosswinds, deviations may increase accordingly.”
Tailwind reports could have similar errors.
The same page says 'the tailwind component is relatively insensitive to FMS errors in the determination of the drift angle'. This suggests that the FMS might indeed provide some useful information. We can all read what we want to out of this learned paper.
What our Brazilian contributor asked was, do any companies encourage its use? The answer is yes...but not in isolation to decide on a go-around. As safetypee says it can provide background or trend info. Focussing on it entirely is wrong, being aware of it can do no harm.
I also find it useful as a trigger to configure a bit earlier, say flaps 10 on the 737, if there is a strong tailwind when descending through 2000 feet. You can cross check this by other cues such as the thrust lever position/N1 when descending with the glideslope. Similar cues are available on short finals.
What our Brazilian contributor asked was, do any companies encourage its use? The answer is yes...but not in isolation to decide on a go-around. As safetypee says it can provide background or trend info. Focussing on it entirely is wrong, being aware of it can do no harm.
I also find it useful as a trigger to configure a bit earlier, say flaps 10 on the 737, if there is a strong tailwind when descending through 2000 feet. You can cross check this by other cues such as the thrust lever position/N1 when descending with the glideslope. Similar cues are available on short finals.
Moderator
Two fairly critical cues are
(a) drift angle for cross wind component
(b) ROD for tailwind
both presuming a steady approach flight path.
Once one has a bit of time on an aircraft one gets a very good feel for when those numbers are suggesting that it is time to go for the miss.
(a) drift angle for cross wind component
(b) ROD for tailwind
both presuming a steady approach flight path.
Once one has a bit of time on an aircraft one gets a very good feel for when those numbers are suggesting that it is time to go for the miss.
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As far as I remember from my dark classroom days the FMC winds are relative to true north ???
However I think most people here are referring to having open Progress page 2 on the FMC which gives wind info relative to aircraft heading so maybe im confusing things too much..
However I think most people here are referring to having open Progress page 2 on the FMC which gives wind info relative to aircraft heading so maybe im confusing things too much..
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I often go to 15kts tailwind runways... and in winter we are often close to that limit...
if we get B/A medium, and medium-poor we will then be close to the limit with regards to landing distance, And data for contaminated runways are CALCULATED...
So yes the last thing i do is to check the FMC for tailwind component to check that i does not read out something stupid...
if we get B/A medium, and medium-poor we will then be close to the limit with regards to landing distance, And data for contaminated runways are CALCULATED...
So yes the last thing i do is to check the FMC for tailwind component to check that i does not read out something stupid...
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FYI, regarding the wind displayed on the ND, on the airbus "The digital direction is with respect to true north, and the analog direction (green arrow) is with respect to magnetic north".
plain-plane, landing in a tailwind on a contaminated runway - its very risky even considering the option – don’t get sucked in.
For JAR-OPS 1, a manufacturer provides contaminated data from the best source, flt test or calculation. However, note the assumptions hidden within this data – depends on reverse, landing in the first 1000ft (Boeing - in a strong tailwind?), at the correct speed, etc, etc.
CS 25 Large Aircraft Amendment 5 AMC 25 1591 (page 665).
For JAR-OPS 1, a manufacturer provides contaminated data from the best source, flt test or calculation. However, note the assumptions hidden within this data – depends on reverse, landing in the first 1000ft (Boeing - in a strong tailwind?), at the correct speed, etc, etc.
CS 25 Large Aircraft Amendment 5 AMC 25 1591 (page 665).
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Regularly pilots ask me about the use of the FMS wind vector. I have written a number of articles that give some background information on this topic. A number of them have been quoted in this forum before. Let me try to share some of my thoughts about this topic.
The FMS wind is not what it appears to be, in particular during takeoff and landing. First, the FMS wind is not corrected to a height of 10 meters. At 500 ft. AGL the wind is about 50% higher than at a height of 10 meters. If the FCOM/AOM gives you a tailwind limit of 10 kts. it applies to this 10 meters AGL (just like the crosswind). The tower wind is normally (but not always!) measured at 10 meters for that reason. Second, internal FMS calculation of especially the crosswind component during approach is filtered, delayed and very sensitive for small errors in track or heading measurement. Furthermore, the FMS wind is not corrected for sideslip. It is true that the FMS tailwind component is relatively insensitive to FMS errors in the determination of the drift angle. However, the problem of an incorrect/inaccurate FMS tailwind does remain due to the height influence and the processing delays. So what should you do when you get a tower report just before landing which significantly differs from the FMS wind vector? The save answer I can give is always to follow the procedure in the FCOM/AOM. This usually will not state that you should use the FMS wind to crosscheck the reported tower wind. At least I am not aware of operators that formally state to use the FMS wind during landing.
On the other hand some airports do not measure the wind accurately. If you believe that this is the case you should report this to your safety department or flight ops department so that they can contact the airport/authorities to take the appropriate actions to solve this problem. And yes, I do realise that in some regions this is a challenging and difficult process.
Some additional reading:
Wind at Your Back - The Hidden Dangers of Tailwind
Aviation Safety Letter 1/2005
Crosswind Certification - How does it affect you?
www.nlr.nl/id~5114/lang~en.pdf
Safety aspects of tailwind operations Safety aspects of tailwind
www.nlr.nl/id~4359/l~en.pdf
Safety aspects of aircraft operations in crosswind
www.nlr.nl/id~4382/l~en.pdf
The FMS wind is not what it appears to be, in particular during takeoff and landing. First, the FMS wind is not corrected to a height of 10 meters. At 500 ft. AGL the wind is about 50% higher than at a height of 10 meters. If the FCOM/AOM gives you a tailwind limit of 10 kts. it applies to this 10 meters AGL (just like the crosswind). The tower wind is normally (but not always!) measured at 10 meters for that reason. Second, internal FMS calculation of especially the crosswind component during approach is filtered, delayed and very sensitive for small errors in track or heading measurement. Furthermore, the FMS wind is not corrected for sideslip. It is true that the FMS tailwind component is relatively insensitive to FMS errors in the determination of the drift angle. However, the problem of an incorrect/inaccurate FMS tailwind does remain due to the height influence and the processing delays. So what should you do when you get a tower report just before landing which significantly differs from the FMS wind vector? The save answer I can give is always to follow the procedure in the FCOM/AOM. This usually will not state that you should use the FMS wind to crosscheck the reported tower wind. At least I am not aware of operators that formally state to use the FMS wind during landing.
On the other hand some airports do not measure the wind accurately. If you believe that this is the case you should report this to your safety department or flight ops department so that they can contact the airport/authorities to take the appropriate actions to solve this problem. And yes, I do realise that in some regions this is a challenging and difficult process.
Some additional reading:
Wind at Your Back - The Hidden Dangers of Tailwind
Aviation Safety Letter 1/2005
Crosswind Certification - How does it affect you?
www.nlr.nl/id~5114/lang~en.pdf
Safety aspects of tailwind operations Safety aspects of tailwind
www.nlr.nl/id~4359/l~en.pdf
Safety aspects of aircraft operations in crosswind
www.nlr.nl/id~4382/l~en.pdf
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That could be a good way to determine the tailwind. I have done this for 42,000 landings in which I took the difference of the QAR recorded GS and TAS at around 10 m. AGL and compared it to the tower wind tailwind at the moment of landing. Correlation between both was good. The problem remains that during the landing as a pilot you want to asssess the tailwind before you have reached 10 meters AGL.
